So, just watched the Episode concerning Twilight's apparent Ascension to Goddesshood. and Star-Swirl's role (indirect or not) in it.
I must say, I'm rather surprised at how much I managed to get right, or at least missed well within the margin of error.
Star-Swirl working on a last spell that is left unfinished, some sort of Interdimensional Void where Alicorns exist but normal ponies cannot, ascention to Alicornism being unheard of...
As to Cadence, I have seen no evidence one way or the other suggesting that she's anything other than a unicorn born with wings due to a one in a million genetic odds. Furthermore, neither Cadence, nor Goddess!Twilight have the large figures of Celestia or Luna, suggesting that while they have Unicorn and Pegasus abilities, it's doubtful that they have Earth Pony in the mix. And if Twilight does (elements of harmony were involved after all), then it isn't 100% like with the Sisters.
Oh, and Alicorn is a thing, so Fanon Haters need to shut up now.